This invention relates to computed tomographic (CT) imaging, and more particularly to methods and apparatus for the detection and diagnosis of lung abnormalities.
In spite of recent advancements in computed tomography (CT) technology, such as faster scanning speeds, larger coverage with multiple detector rows, and thinner slices, energy resolution is still a missing piece. Namely, wide x-ray photon energy spectrum from the x-ray source and the lack of energy resolution from CT detection systems preclude energy discrimination CT.
X-ray attenuation through a given object is not a constant. Rather, the X-ray attenuation is strongly dependent on the x-ray photon energy. This physical phenomenon manifests itself in the image as beam-hardening artifacts, such as, non-uniformity, shading, and streaks. Some beam-hardening artifacts can be easily corrected, but other beam-hardening artifacts may be more difficult to correct. In general, known methods to correct beam hardening artifacts include water calibration, which includes calibrating each CT machine to remove beam hardening from materials similar to water, and iterative bone correction, wherein bones are separated in the first-pass image then correcting for beam hardening from the bones in the second-pass. However, beam hardening from materials other than water and bone, such as metals and contrast agents, may be difficult to correct. In addition, even with the above described correction methods, conventional CT does not provide quantitative image values. Rather, the same material at different locations often shows different CT numbers.
Another drawback of conventional CT is a lack of material characterization. For example, a highly attenuating material with a low density can result in the same CT number in the image as a less attenuating material with a high density. Thus, there is little or no information about the material composition of a scanned object is based solely on the CT number. At least some state-of-the-art CT scanners currently available are limited to providing anatomical information. For lung scans, images produced by such scanners exhibit a significant level of image artifacts and CT number inaccuracy. These limitations prevent the utilization of the CT device for advanced diagnosis. Accordingly, the methods and apparatus described herein address the detection and diagnosis of lung abnormalities.